(a) Field of the Invention
The present invention relates to a superconductive material with an electrical resistance that disappears at a low temperature temperature of which is more than the boiling point of 20.3.degree. K. (-252.7.degree. C.) of liquid hydrogen. More specifically, the present invention relates to La-Ba-Cu-O series superconductive material.
(b) Description of the Related Art
Hitherto known superconductors cannot avoid cooling by cryogenic liquid helium (boiling point: 4.2.degree. K.), and the vast popularization has therefore been prevented by the expensive cooling cost and the uneven distribution of helium in resources.
A confirmed superconductive material having the highest critical temperature is Nb.sub.3 Ga. Its critical temperature T.sub.c (i.e. transition starting temperature) is 23.6.degree. K., and the material cannot reach a usable level under cooling of liquid hydrogen (i.e. boiling point: 20.3.degree. K.) or liquid neon (i.e. boiling point: 27.1.degree. K.).
In general, in order to confirm a certain material as superconductive, it is necessary to certify that (1) the material structure is definite, (2) the material has a stability for superconductivity and the test result is reproducible, (3) an electrical resistance is rapidly lowered from a certain temperature (i.e. superconductivity transition starting temperature) together with the temperature drop within a range of several degrees, and (4) the material shows the Meissner effect (i.e. complete diamagnetism) characteristic of a superconductive phenomenon.
Thereto, no material satisfying any one of the above conditions can be found, and there has not been developed any practically usable superconductive material at a temperature higher than the boiling point of liquid hydrogen.